Provide a robust O(1) mechanism to check for infinite iterators

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-06-22.12:48:14.537>
labels = ['type-feature', '3.8']
title = 'Provide a robust O(1) mechanism to check for infinite iterators'
updated_at = <Date 2018-09-04.01:06:44.169>
user = 'https://github.com/ncoghlan'

bugs.python.org fields:

activity = <Date 2018-09-04.01:06:44.169>
actor = 'koos.zevenhoven'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2018-06-22.12:48:14.537>
creator = 'ncoghlan'
dependencies = []
files = []
hgrepos = []
issue_num = 33939
keywords = []
message_count = 19.0
messages = ['320229', '320232', '320234', '320235', '320237', '320239', '320241', '320297', '320323', '320350', '320352', '320554', '320567', '320568', '320580', '320581', '320589', '323666', '324548']
nosy_count = 8.0
nosy_names = ['rhettinger', 'ncoghlan', 'pitrou', 'erik.bray', 'serhiy.storchaka', 'jdemeyer', 'koos.zevenhoven', 'bbayles']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue33939'
versions = ['Python 3.8']